chore: improve aiosqlite maintenance path

[lphuc2250gma]

Summary:

• Add a focused unit test in aiosqlite/tests/smoke.py covering async iteration over a Cursor after fetchmany() has partially consumed rows (boundary between partial fetch and async-for continuation), and add/tighten return-type annotations on Cursor methods in aiosqlite/cursor.py where currently inferred or missing.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.